test/ssl/bufferevent_wm: explicitly break the loop once client/server received enough

test/ssl/bufferevent_wm: explicitly break the loop once client/server received enough

There can be tricky cases (that can be reproduced by reducing
SO_RCVBUF/SO_SNDBUF to 6144, on linux, and be aware, since linux doubles
this const), when there is still write event pending, although we read
enough.

This should be fixed in a more sophisticated way, but to backport the
patch, let's simply break the loop manually.

The ssl/bufferevent_wm originally failed on solaris.